fix: don't error on slot prop inside block inside other component

[dummdidumm]

slot is treated as a regular prop if it is not used directly inside another component, but we were running validations on such regular props that should only be run on real slots.

Fixes #15125

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Latest commit: 45eebc7

The changes in this PR will be included in the next version bump.

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pnpm add https://pkg.pr.new/svelte@15148